Can you help me troubleshoot this problem? (picture inside)

Discussion in 'General Electronics Chat' started by pureanalog, Nov 11, 2012.

  1. pureanalog

    Thread Starter Member

    Oct 8, 2010
    36
    0
    This is the circuit I am working on. It is a guitar Digital Signal Processing preamp. The unit has two independent set of outputs (stereo sets). OUTPUT 1 which has both XLR and RCA jacks and OUTPUT 2.

    The problem is that the OUTPUT 1, is not working on the left channel both on the XLR and the unbalanced out jacks.

    I did a few tests with my multimeter.

    D4 and D16 on their bottom sides are short circuited between each other (they correspond to the channel that doesn't work), whereas D13 and D16 don't behave like that. D4 doesn't measure like a diode while ON the circuit. Its equivalent D16 which is on the right channel that works like a diode while ON the circuit.

    The exact same symptoms I notice on D10 and D8 whereas everything seems normal on D20 and D19.


    Also the bottom sides of C280 and C270 are short circuited between them. There is an equivalent set of capacitors on the RIGHT channel on the XLR output which cannot be seen on the picture, but the bottom sides of those caps are not short circuited between them.

    [​IMG]

    Unfortunately I do not have the schematics for the unit , nor are they available.

    Any ideas on how to troubleshoot further since there are no schematics available.

    Unfortunately the company is located too far away to be able to ship there for repair.
     
  2. timescope

    Member

    Dec 14, 2011
    298
    44
    Unsolder one side of each of the diodes that are short circuited and check them. If the diodes are ok, check the ics U11 and U12. You can do a google search for the ic datasheet .
    Timescope
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    You can measure them when the parts are on board.

    1. Measuring when the Power OFF, using the multimeter and turn to resistor range, comparing the values of the parts of left side and right side, and to check any values is difference, measuring between themself and measuring them between GND.

    2. Measuring when the Power ON, using your fingers to test the heat of parts, and be careful, if the heat is too hot, then you can comparing left and right.

    3. Turn the multimeter to the Voltage range, measuring the parts as item 1.
     
  4. pureanalog

    Thread Starter Member

    Oct 8, 2010
    36
    0
    thanks for the replies.

    I measured D10 diode out of the circuit and it doesn't seem to work. It is short circuited both ways. Do you think that can be causing the no-sound problem for the left channel?

    Also is there an easy way to identify the properties and characteristics of the diode to buy a similar one?
     
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Does any IC on the board or just transistors?
    Except the diode is a Zener, otherwise, you can use 1N4148 to try, or you can De-soldering the diode which mirror to right channel, and measure it is a zener or not.

    You can add about 1K resistor to the diode which De-soldering from right channel, and add 5V to the two terminal, change the direction of the diode, and measure the voltages of the diode.

    It depends on where is the diode located.
    If you can draw the circuit of the board, maybe it will more easier to solve your problem.
     
    Last edited: Nov 14, 2012
  6. pureanalog

    Thread Starter Member

    Oct 8, 2010
    36
    0
    thanks for your help guys. It was the diodes fault. Everything is back to normal.
     
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